1. Field of the Invention
The present invention relates to a power breaker system which selectively breaks only a fault circuit formed by a load shortcircuit etc. in a power system.
2. Description of the Prior Art
Heretofore, it has been known to use the system shown in FIG. 1 wherein the reference (M) designates a main breaker for wiring which is connected to a power source at one terminal; (F.sub.1), (F.sub.2) . . . (F.sub.n) respectively designate feeder circuit breakers for wiring which are respectively connected between the other terminal and each of the loads.
In the conventional power breaker system shown in FIG. 1, when a short-circuit fault is caused in one load circuit having a feeder circuit breaker (F.sub.1), it is necessary to actuate only the feeder circuit breaker (F.sub.1) before actuating the main breaker (M) in order to continue the feeding to the non-fault load circuit while breaking only the fault load circuit.
Thus, the main breaker (M) has a structure for breaking by a large breaking current larger than the breaking current for the feeder circuit breakers (F.sub.1), (F.sub.2) . . . (F.sub.n) or by said large current and a predetermined delay time.
A bimetal thermal actuating type or an electromagnetic type trip mechanism has been used as the conventional trip mechanism of the main breaker for the short-circuit current. However, in any conventional system, it has been considered to be impossible to break selectively the circuit in all fault current ranges. That is, the operating characteristics of the main breaker (M) and the feeder circuit breaker (F.sub.1) under a short-circuit fault in a load (not shown) connected to the feeder circuit breaker (F.sub.1) are shown in FIG. 2. In the large fault current range, the main breaker (M) is simultaneously actuated together with the feeder circuit breaker (F.sub.1) whereby the selective breakable range is limited to only a small fault current range. It has been known to use a power breaker system shown in FIG. 3 which has a trip timing characteristic of the main breaker (M) to cause a delay actuation whereby the selective breaking has been attained for certain fault current range. However, the selective breakable range has been small as a current of about 10 KA to 20 KA. Thus, the industrial value of such system is not high enough in practical use.
It has been considered to provide a system shown in FIG. 4 for selectively breaking in a large current range. However, the total breaking time t of the main breaker (M) is prolonged to be over the allowable limit I.sup.2 t (I: breaking current) as the durability of the main breaker (M). In order to keep the allowable limit I.sup.2 t, the breaking capacity (that is t) of the main breaker (M) should be decreased to reduce the industrial value in the practical use.